In the field of sonar, a transducer is employed in the detection of underwater objects and is either a transmitter or a receiver. A projector is a sonar transmitter utilized to convert electrical signals to mechanical vibrations while a receiver intercepts reflected signals. Projector transmitters and receivers are known and separate projector and receiver arrays are formed from multiple projectors and receivers, respectively. The arrays are then utilized in conjunction with a sea craft to detect the underwater objects.
A projector generally comprises an electromechanical stack element that converts electrical signals to mechanical vibrations. The stack element can be comprised of ceramic having a particular crystal structure. Ceramic projectors must be operated in an optimal temperature range to provide good performance. Further, ceramic projectors are normally operated in one of two operating regions depending upon the ceramic crystal structure. The two operating regions include the piezoelectric region and the electrostrictive region.
If the ceramic crystal is subjected to a high direct current voltage during the manufacturing process, the ceramic crystal becomes remanent polarized and operates in the piezoelectric region. The electrical signal is then applied to the ceramic stack to generate mechanical vibrations. As an alternative, direct current voltage can be temporarily applied to the ceramic stack during operation to provide polarization of the crystal. Under these conditions, the operation of the projector is in the electrostrictive region. After the application of the direct current voltage is discontinued, the ceramic stack is no longer polarized.
Many different types of sonar projectors are known. One particular type of projector is identified as a flextensional sonar projector which is a low frequency transducer. The low frequency transducer exhibits low attenuation of the acoustic signals in sea water. In general, a ceramic stack is housed within an elliptical-shaped outer projector shell. Vibration of the ceramic stack caused by application of an electrical signal produces magnified excursions in the outer projector shell. Thereafter, the excursions generate acoustic waves in the sea water. By way of example, one form of a flextensional transducer for underwater use can be found in PCT International Publication Number WO 87/05772.
A second type of sonar projector is known as the slotted cylinder projector. In the slotted cylinder projector, at least one ceramic stack or cylinder is enclosed within an outer cylindrical shell. A slice of the outer cylindrical shell and the ceramic cylinder are removed to form a slot. The vibrations of the ceramic cylinder are transferred to the edges of the outer cylindrical shell bordering the slot. The mechanical vibrations thereafter generate the acoustic waves in the sea water. A third type of sonar projector is the longitudinal vibrator projector which sandwiches the ceramic material between a head and a tail portion. The mechanical vibrations generated by the ceramic material are transmitted through the projector head.
Each of the above-described sonar projectors are known and, in general, utilize a ceramic material identified as PZT ceramic. PZT ceramic is a dense heavy material. Thus, an array of projectors each having a ceramic stack fashioned from PZT is extremely heavy (e.g., 30-40 tons). Therefore, a major problem associated with projector arrays of the prior art used to detect underwater objects is the weight of the array. Large amounts of energy must be expended to drag the projector arrays of the prior art utilizing PZT ceramic material through a body of water.
Other problems exist when using PZT ceramic material. In a slotted cylinder projector, the PZT ceramic material positioned within the outer cylindrical shell experiences high compressive stresses. The high compressive stresses cause the PZT ceramic material to become depolarized, e.g., to loose the remanent polarization. The polarization of the ceramic crystal is necessary to enable the applied electrical signal to generate the mechanical vibrations within the stack. Depolarization results in loss of the piezoelectric properties. Thus, the PZT ceramic material fails to function properly when exposed to the high compressive stresses.
Another known ceramic material suitable for fashioning a projector stack is lead magnesium niobate-lead titanate, hereinafter referred to as PMN-PT. Use of PMN-PT ceramic as the driver to generate mechanical vibrations in a sonar projector has been attempted. The PMN-PT ceramic material exhibits high electrostrictive activity. Therefore, use of the PMN-PT ceramic to fashion a sonar projector stack is attractive since a substantial increase in acoustic output signal is potentially available.
The characteristics of PMN-PT ceramic vary as a function as temperature. Therefore, it is essential that the thermal design of a projector utilizing PMN-PT material be stable. Stability must be achieved by maintaining the projector ceramic material close to a predetermined temperature. If the PMN-PT ceramic material is not operated within the predetermined temperature range, the dynamic acoustic electrostrictive characteristics of the ceramic material will decrease. A decrease in the electrostrictive characteristics of the ceramic material results in reduced performance of the sonar projector.
The affected dynamic acoustic electrostrictive characteristics of the PMN-PT material include strain, coupling and dielectric. In the art, the term strain is defined as the change in length of the ceramic stack over the original length that occurs as a result of applying an electric field to the stack. The term coupling is defined as the ability of the projector to transform electrical energy to mechanical energy. Finally, the term dielectric is defined as the potential power (either piezoelectric or electrostrictive) of the ceramic material.
Prior attempts to build a sonar projector having a ceramic stack comprised of PMN-PT material are known. This effort has been concentrated on lowering the internal losses in the crystal structure and in reducing the duty cycle of the projector. The internal losses are voltage type losses which tend to generate heat in the ceramic structure. For example, in a projector array developing (50-100) KW, the voltage type losses are substantial. The duty cycle of the projector is the percent of time during the complete cycle that the projector is transmitting. That portion of the duty cycle in which the projector is not transmitting is a projector "cool down" time. This procedure permits the temperature of the PMN-PT material to be stabilized close to the ambient temperature. Unfortunately, the procedure has proved to be somewhat impractical due to the inherent heat generation of very high powered sonar projectors and to inefficiency. The duty cycle was kept low to avoid heating effects.
The power output level of a sonar projector is high only within a certain temperature range. The ceramic material of the PMN-PT projectors of the prior art were formulated to operate at room temperature. This formulation provided lower internal losses and minimized temperature increases in the ceramic material. Unfortunately, the power generated caused the temperature of the projector to increase. The increased projector temperature exceeded the predetermined temperature range which resulted in a reduced the output signal.
Thus, a need remains in the art for an improvement in conventional sonar projectors for increasing the power level and duty cycle while simultaneously decreasing the size and weight.